This invention relates generally to an electron gun such as used in a cathode ray tube for providing a video image on a display screen and is particularly directed to an improved electron beam forming region in an electron gun for reducing spherical aberration in the electron beam and improving video image resolution.
Electron guns employed in cathode ray tubes (CRTs) generally can be divided into two basic sections: (1) a beam forming region (BFR) and (2) an electron beam focus lens for focusing the electron beam on the phosphor-bearing screen of the CRT. The electrons emitted from a cathode are directed toward the BFR and formed into a beam bundle and are further directed through a main lens region. The BFR typically is comprised of a G1 control grid, a G2 screen grid and a portion of a G3 grid in facing relation with the G2 screen grid. The energetic electrons are directed through aligned apertures in these three grids and are thereby formed into a well-defined beam having a very small, circular cross section. The beam is focused to a small spot on the CRT""s display screen and is deflected in a raster-like manner at very high speeds to form a video image on the display screen. On the case of a color CRT, three electron beams are simultaneously formed, focused, and are converged to a single spot on the display screen. The three electron beams are then displaced in unison in a raster-like manner over the display screen in forming a color video image.
Referring to FIG. 1, there is shown a simplified sectional view of a beam forming region (BFR) 88 in a typical prior art electron gun. BFR 88 includes a cathode 90 which emits energetic electrons along an axis A-Axe2x80x2 in the direction of a G1 control grid 92. Axis A-Axe2x80x2 represents the longitudinal axis of the electron gun. The energetic electrons are directed through a circular aperture 92a in the G1 control grid 92. The energetic electrons then transit an aperture 94a in a G2 screen grid 94 and thence are directed through an aperture 96a in a G3 grid 96. While the G1 control grid 92 and the G2 screen grid 94 are each typically in the form of a flat plate, the G3 grid is typically tubular in shape and extends to the left of the figure although this is not shown in the figure for simplicity. Thus, element 96 in FIG. 1 represents only the low voltage side of the typical G3 grid.
Each of the G1 control, G2 screen and G3 grids 92, 94 and 96 is charged to a predetermined voltage for forming electrostatic fields through which the energetic electrons are directed for forming the electrons into a narrow beam. The electrostatic field produced by the G2 screen grid 94 in the area of its aperture 94a and along axis A-Axe2x80x2 is shown by a series of spaced curvilinear lines. These curvilinear lines are known as equipotential lines, each having the same electrostatic potential value along its length. From the figure, it can be seen that the charged G2 control grid 94 forms equipotential lines which bend inwardly toward the center of the grid in the vicinity of its aperture 94a. The electrostatic field represented by the field vector {right arrow over (E)} applies a force represented by the force vector {right arrow over (F)} to an electron where {right arrow over (F)}=-e{right arrow over (E)}, where xe2x80x9cexe2x80x9d is the charge of an electron. The electrostatic field and force vectors are oriented perpendicular to the equipotential lines and are opposite in direction. The low voltage side of the G2 screen grid 94, i.e., the portion of the G2 screen grid in facing relation to the G1 control grid 92, operates as a diverging lens. The high voltage side of the G2 screen grid 94, i.e., the portion of the grid in facing relation to the G3 grid, functions as a converging lens to effect electron beam crossover of axis A-Axe2x80x2. This is shown by dotted lines 98 and 100 which represent two electron trajectories in transiting BFR 88. From the figure, it can be seen that as the electron beam transits the electrostatic field of the G2 screen grid 94 in facing relation to the G1 control grid 92, the electrons are directed away from axis A-Axe2x80x2 in a diverging manner. The electrons in the beam continue to diverge away from axis A-Axe2x80x2 until they encounter the electrostatic field of the G2 screen grid 94 in facing relation to the G3 grid 96, whereupon the electrons are subjected to a converging force which directs the electrons toward axis A-Axe2x80x2 as shown on the left side of FIG. 1. The electrons continue to converge toward axis A-Axe2x80x2 until they cross over the axis in the focusing region of the electron gun.
The converging electrostatic field of the G2 screen grid 94 in facing relation to the G3 grid 96 exerts a strong converging force on the electron beam as the individual electrons are directed back toward axis A-Axe2x80x2. This strong convergence lens effect on the high side of the G2 screen grid 94 gives rise to spherical aberration of the electron beam and causes a large beam spot on the CRT""s display screen and a reduction in video image definition and resolution.
The present invention addresses the aforementioned limitations of the prior art by providing an improved beam forming region for an electron gun which maintains a strong beam divergence effect while at the same time reduces the convergence force applied to the electron beam as it is formed resulting in a corresponding decrease in electron beam spherical aberration and improved definition and resolution of the video image produced by the electron beam.
Accordingly, it is an object of the present invention to provide an improved beam forming region in an electron gun for a cathode ray tube which reduces the spherical aberration of an electron beam on the cathode ray tube""s display screen.
It is another object of the present invention to provide in a single- or multi-beam cathode ray tube a variable sized beam, passing aperture in the G2 screen grid of the cathode ray tube""s electron gun to maintain strong electron beam divergence while reducing the convergence lens effect on an electron beam produced by the electron gun and the resulting spherical aberration of the final beam spot.
A further object of the present invention is to reduce the strength of an electrostatic convergence lens on an electron beam in the beam forming region of an electron gun in a cathode ray tube.
It is yet another object of the present invention to provide an electron gun incorporating a beam forming region which reduces electron beam spherical aberration for improved video image resolution particularly when used with high electron beam currents as in an electron gun for a projection television receiver.
This invention contemplates a beam forming arrangement in an electron gun for forming energetic electrons provided by a cathode into an elongated beam having a small cross section, the beam forming arrangement comprising a G1 control grid disposed adjacent the cathode and including a first aperture through which the energetic electrons are directed; a lower portion of a G3 grid having a third aperture aligned on a common axis with the first aperture in the G1 control grid; and a G2 screen grid disposed intermediate the G1 control grid and the G3 grid and having a second aperture aligned on the common axis with the first and third apertures, wherein energetic electrons directed through the first aperture transit the second and third apertures in forming a beam of electrons, and wherein the second aperture is defined by a first opening in the G2 screen grid in facing relation to the G1 control grid having a diverging lens effect on the electron beam and a second opposed opening in the G2 screen grid in facing relation to the lower portion of the G3 grid having a converging lens effect on the electron beam, and wherein the second opening is greater than the first opening.